This invention relates to a positron computed tomography (CT) scanner.
Positron CT scanners, also referred to as PET (Positron Emission Tomography) are known (S. E. Drenzo et., IEEE Trans. Nucl. Sci., NS-26, No. 2: 2790-2793, 1979, etc.). These devices are used for dosing medicines to patients, and such medicines are labelled by radioactive isotopes to image their internal distributions in the patients. In such PET, a number of .gamma. (gamma) ray detectors are arranged in rings for the detection by coincidence counting of a pair of annihilating photons (0.51 MeV) emitted in directions opposite to each other by the pair annihilation of a positron and an electron. Multi-layers of detector rings are provided, and an intra-ring image can be formed by means of the coincidence counting of the detectors belonging to one ring, while an inter-ring image can be obtained by means of the coincidence counting of adjacent ones of the rings. Accordingly, a PET scanner having j layers of detector rings can provide tomographic images of 2j-1 layers at once. A slice septum (collimator) is provided between one of the detector rings and its adjacent one for shielding unnecessary .gamma. rays and internal scattered .gamma. rays, whereby unnecessary counting by the detectors is decreased to reduce counting losses and accidental coincidences. The resolving power is determined mostly by an area of a photon incident surface of a detector (BGO scintillator). The sectional resolving power of the current devices has reached 3-5 mm (full width at half maximum value).
FIG. 1 is a schematic sectional view of a conventional PET scanner. Each .gamma. ray detector 110 includes both a scintillator 111 and a photomultiplier 112 and these detectors are arranged in rings laid one above another, and shield collimators 120 are provided above the top ring and below the bottom ring. The central axis of the rings is shown as X in FIG. 1. Slice septa 130 are arranged so as to restrict the detection fields of the .gamma. ray detectors 110 vertically to the central axis. Generally one slice septum 130 is provided per one layer of .gamma. ray detectors 110. The .gamma. ray shielding effect of the slice septa 130 can reduce noise components as described above.
That is, in the case of coincidence counting by two .gamma. ray detectors 110, as shown in FIG. 1, a pair of .gamma. rays from one radioisotope 141 are incident on the .gamma. ray detectors 110.sub.1, 110.sub.2 ; .gamma. rays from two radioisotopes 142, 143 are concurrently incident by accident on the .gamma. ray detectors 110.sub.3, 110.sub.4 ; or one of a pair of .gamma. rays from one radioisotope 144 being detected by the .gamma. ray detector 110.sub.6, the other of the .gamma. rays being scattered by a scatterer 150 and detected by the .gamma. ray detector 110.sub.5. All the coincidences other than those of the .gamma. rays from the radioisotope 141 are noise. The slice septa 130 for shielding .gamma. rays restrict the detection fields of the .gamma. ray detectors 110 to reduce such noise components.
But as the respective .gamma. ray detectors 110 are increasingly more miniaturized to make an interval between each of the slice septa 130 and its adjacent one narrower for the improvement of a resolving power, the .gamma. ray detecting efficiency is lowered proportionally to a square of an interval between the slice septa 130. Accordingly it tends to take much more time for counting to form a sufficiently precise image. As a countermeasure to this, so-called three-dimensional PET has been recently studied. In the three-dimensional PET, no slice septa are provided to count not only intra-coincidences of respective detector rings and inter-coincidences between adjacent ones of the detector rings, but also coincidences between ones of the detector rings remote from each other are counted to obtain images at respective slices to restore an image.
However, such three-dimensional PET, which acquires and processes three-dimensional data, has a disadvantage of the above-mentioned quite high noise because slice septa are not used. In the case such three-dimensional PET including the conventional slice septa provided between detector rings is realized, the numerical aperture of the .gamma. ray detectors 110 is comparatively large, and inevitably the slice septa 130 have to be thinned and small-sized. Then the .gamma. ray shielding effect of the slice septa 130 is lowered with the result that the above-mentioned accidental coincidences and scattered coincidences are adversely acquired in data, and sharp image cannot be obtained.